The molecular biology of the flavin-containing monooxygenases of man

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Abstract

cDNA clones encoding five distinct members of the FMO family of man (FMOs 1,2, 3, 4 and 5) were isolated by a combination of library screening and reverse transcription-polymerase chain reaction techniques. The deduced amino acid sequences of the human FMOs have 82–87% identity with their known orthologues in other mammals but only 51–57% similarity to each other. The hydropathy profiles of the proteins are very similar. From the calculated rate of evolution of FMOs (a 1% change in sequence per 6 million years) it would appear that individual members of the FMO gene family arose by duplication of a common ancestral gene some 250–300 million years ago. Each of the FMO genes was mapped by the polymerase chain reaction to the long arm of human chromosome 1. The localization of the FMO1 gene was further refined to 1q23–q25 by in situ hybridization of human metaphase chromosomes. RNase protection assays demonstrated that in man each FMO gene displays a distinct developmental and tissue-specific pattern of expression. In the adult, FMO1 is expressed in kidney but not in liver, whereas in the foetus its mRNA is abundant in both organs. FMO3 expression is essentially restricted to the liver in the adult and the mRNA is either absent, or present in low amounts, in foetal tissues. FMO4 is expressed more constitutively. Human FMO1 and FMO3 cDNAs were functionally expressed in prokaryotic and eukaryotic cells. FMO1 and FMO3, expressed in either system, displayed product stereoselectivity in their catalysis of the N-oxidation of the pro-chiral tertiary amines, N-ethyl-N-methylaniline (EMA) and pargyline. Both enzymes were stereoselective with respect to the production of the (−)-S-enantiomer of EMA N-oxide. But in the case of pargyline, the enzymes displayed opposite stereoselectivity, FMO1 producing solely the (+)-enantiomer and FMO3 predominantly the (−)-enantiomer of the N-oxide.

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    Citation Excerpt :

    It is one of the most important non-cytochrome P450 (CYP) enzymes involved in the metabolism of endogenous substances and xenobiotics containing nucleophilic nitrogen, sulfur, and phosphorous heteroatoms [3–5]. Humans possess five functional FMO genes, designated FMO1–5 [6,7]. FMO1–4 are clustered on chromosome 1, in the region q24.3, and FMO5 is located at 1q21.1 [7].

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